In a storage device such as a disk array apparatus, a method which creates complete replication of backup target volume is known as a method to create data backup of backup target volume in a remote site. This method needs a backup volume having the same capacity as the backup target volume. Accordingly, when a user wishes to create and manage multiple generations of backup volumes in a remote site, a large amount of volumes are needed and the backup cost increases.
In order to avoid such demerits of backup by the complete replication, a data backup method using a snapshot method is known (for example, patent document 1 (Japanese Patent Application Publication No. 2004-192133), patent document 3 (Japanese Patent Application Publication No. 2008-004090) and patent document 4 (Japanese Patent Application Publication No. 2010-092176)). This method can create the backup volume with small amount of necessary storage capacity compared with the method to create the complete replication.
The snapshot method mentioned here is the technology for maintaining the state of the backup target volume (the master volume) at a time of creating the snapshot. For example, supposing a snapshot of the master volume which stores data shown in FIG. 13 has been created, the snapshot volume having the same storage capacity as this master volume is created. And, for example, at the time when writing of data “EE” occurs in a storage area of data “BB” of the master volume, the data “BB” before updated is stored in the identical address of the snapshot volume (FIG. 14).
In other words, the backup target volume is instantly replicated seemingly because it is enough only defining the snapshot volume at the time of snapshot creation.
When a host reads data of the snapshot volume, the data of the snapshot volume is referred to for the address in which data exists in the snapshot volume, and the data of the master volume is referred to for the address in which data does not exist in the snapshot volume. As a result, the host can acquire the data of the master volume as of snapshot creation.
A method which creates multiple generations of snapshot volumes in a remote site using this snapshot method exists (for example, patent document 2 (Japanese Patent Application Publication No. 2005-292865)). This method can create the multiple generations of backup volumes in a remote site with suppressing the backup cost.
However, snapshot volume access by the snapshot method refers to the master volume when data does not exist in the snapshot volume. Therefore, the method described in the patent document 2 needs the replication volume for the master volume in the remote site (refer to FIG. 15). Also, it needs to synchronize the replication volume with the master volume when creating the snapshot volume because the snapshot volume access refers to the replication volume in a case where data does not exist in the snapshot volume. As mentioned above, in the above-mentioned technology, it is necessary to synchronize the replication volume with the master volume at the time of snapshot creation.